WO2022157858A1

WO2022157858A1 - Wavelength conversion apparatus

Info

Publication number: WO2022157858A1
Application number: PCT/JP2021/001862
Authority: WO
Inventors: 貴大柏崎; 拓志風間; 晃次圓佛; 修忠永; 飛鳥井上; 信建小勝負; 亮一笠原
Original assignee: 日本電信電話株式会社
Priority date: 2021-01-20
Filing date: 2021-01-20
Publication date: 2022-07-28
Also published as: JP7473850B2; JPWO2022157858A1

Abstract

The present invention allows the maximum intrinsic performance of a nonlinear optical element to be stably exhibited regardless of surrounding environments and methods of use. Provided is a wavelength conversion apparatus (100) including a nonlinear optical device (101) comprising a secondary nonlinear optical element that generates conversion light of different wavelengths from one or more fundamental wave light; and a temperature control device that controls the temperature of the element of the nonlinear optical device. The wavelength conversion apparatus is provided with: a means (102) that inputs monitor light of a wavelength different from the wavelength of the fundamental wave light to the nonlinear optical device; a light detection means (103, 106) that separates the monitor light outputted from the nonlinear optical device, and detects light intensity; and a control means (104, 105) that controls the temperature control device on the basis of the light intensity detected by the light detection means.

Description

Wavelength converter

The present invention relates to a wavelength conversion device, and more particularly to a wavelength conversion device that includes an optical element having a nonlinear optical effect and is applicable to optical communication systems, optical measurement systems, and the like.

For applications such as optical signal wavelength conversion in optical communication, optical modulation, optical measurement, optical processing, medical treatment, biotechnology, etc., it is possible to generate and modulate coherent light over the ultraviolet, visible, infrared, and terahertz regions. Non-linear optical devices and electro-optical devices are being developed.

Various materials have been researched and developed as nonlinear optical media and electrooptic media used in such optical devices. Among them, oxide-based compound substrates such as lithium niobate (LiNbO ₃ ) are known as promising materials because of their extremely high secondary nonlinear optical constants and electro-optic constants. Periodically poled lithium niobate (PPLN) is known as an example of an optical device using lithium niobate with high nonlinearity.

In the second-order nonlinear optical effect, signal light with wavelength λ1 and pumping light with wavelength λ2 are input to a second-order nonlinear medium to generate new converted light with wavelength λ3. First,
1/λ3=1/λ1+1/λ2 Formula (1)
A wavelength conversion operation that satisfies is called sum frequency generation (SFG). Second, let λ1=λ2 and
λ3=λ1/2 Formula (2)
A wavelength conversion operation that satisfies is called second harmonic generation (SHG). Third,
1/λ3=1/λ1−1/λ2 Formula (3)
A wavelength conversion operation that satisfies is called difference frequency generation (DFG). By such a wavelength conversion operation, converted light having different wavelengths is generated from one or a plurality of fundamental wave lights.

For example, in the mid-infrared wavelength region of 2 to 5 μm, there are strong absorption lines such as the normal vibration of various environmental gases, so the development of a compact mid-infrared light source is desired. DFG is considered promising for such a light source in the mid-infrared region because it can use a technologically mature pumping light source of around 1 μm and signal light in the communication wavelength band.

In addition, since there is a wavelength region of visible light around 0.5 μm that is difficult to achieve with a semiconductor laser, an excitation light source of around 1 μm is used to generate visible light such as green light by SHG or SFG. Wavelength conversion technology that can generate light is promising.

By using wavelength conversion technology using DFG, it is possible to collectively convert light in the 1.55 μm wavelength band, which is mainly used in optical fiber communication, into another wavelength band. For this reason, it can be applied to optical routing in wavelength division multiplexing, collision avoidance of wavelengths in optical routing, etc., and wavelength converters are considered as one of the key devices for constructing large-capacity communication optical networks. there is

Also, in wavelength conversion using DFG, signal distortion compensation can be performed using the fact that the converted light becomes phase conjugate light with respect to the signal light. When the signal light is converted to phase conjugate light at approximately the midpoint of the transmission line, the signal distortion caused by the dispersion occurring in the transmission line before conversion and the nonlinear optical effect in the fiber cancel each other out in the transmission line after conversion. propagates as As a result, it is considered as one of the key devices that can reduce dispersion and nonlinear signal distortion.

By using a wavelength conversion element with high wavelength conversion efficiency, it is possible to configure a signal light amplifier called optical parametric amplification by transferring energy from pumping light power to signal light. In particular, a phase sensitive amplifier, which has amplification characteristics corresponding to the phase relationship between pump light and signal light, is expected as a technology capable of low-noise optical amplification.

In addition, the degenerate optical parametric amplification process can generate photon pairs with quantum correlation, and can generate non-classical states such as the generation of squeezed light and single photon states with messengers. These lights are expected to be important resources for optical quantum computers and sensing technology using quantum light.

Optical waveguide devices are effective in obtaining highly efficient and broadband nonlinear optical effects in PPLN. This is because the wavelength conversion efficiency is proportional to the power density of light propagating through a nonlinear medium, and by forming a waveguide structure, light can be confined within a limited range. Therefore, various waveguides using nonlinear media have been researched and developed. So far, studies have been made using diffusion type waveguides called Ti diffusion waveguides and proton exchange waveguides.

However, these waveguides have problems in terms of optical damage resistance and long-term reliability because impurities diffuse into the crystal during the fabrication process. In diffusion type waveguides, the optical power that can be input to the waveguide is limited because the crystal is damaged due to the photorefractive effect when high-intensity light is incident on the waveguide.

In recent years, ridge-type optical waveguides with features such as high optical damage resistance, long-term reliability, and easy device design have been researched and developed because the bulk characteristics of crystals can be used as they are. As a method for producing a ridge-type waveguide, a ridge-type optical waveguide can be formed by bonding two substrates with an adhesive, thinning one of the substrates, and performing ridge processing.

However, the method of bonding the substrates together with an adhesive has the problem that the thin film cracks when the temperature changes due to the difference in thermal expansion coefficient between the adhesive and the substrate. In addition, since the second harmonic light generated in the waveguide deteriorates the adhesive, the waveguide loss increases during operation and the efficiency of wavelength conversion deteriorates. Furthermore, there is also a problem that the thickness of the single crystal film becomes uneven due to the nonuniformity of the adhesive layer, and the phase matching wavelength of the wavelength conversion element shifts.

Therefore, direct bonding technology is known as a technology for firmly bonding substrates together without using an adhesive. The direct bonding method is a method in which wafers that have been surface-treated using chemicals are placed on top of each other and bonded by an attractive force between the surfaces. Bonding is performed at room temperature, but since the bonding strength of the wafers at this time is small, heat treatment is performed at a high temperature to improve the bonding strength. In addition to its features such as high optical damage resistance, long-term reliability, and ease of device design, direct bonding technology avoids contamination of impurities and absorption of adhesives, etc. in the generation of light in the mid-infrared region by the above-mentioned DFG, for example. It is also promising from the point of view that it can be done.

Moreover, direct bonding technology is expected to be applied not only to nonlinear optical devices but also to high-power optical modulators. Oxide-based compound substrates such as lithium niobate (LiNbO ₃ ) have large electro-optic constants in addition to second-order nonlinear optical constants, and are widely used as optical modulators using the electro-optic effect (EO effect). However, although optical modulators using Ti-diffused waveguides have been commercially available, it has been difficult to input high-power light of 100 mW or more. The use of direct bonding technology enables watt-class optical input, so it is expected to be applied to the generation of optical modulation signals with high optical intensity and laser processing technology.

Since the direct bonding method requires a heat treatment at a high temperature of about 400° C., the wafers to be bonded are required to have good surface flatness and similar coefficients of thermal expansion. For this reason, direct bonding of lithium niobate (LiNbO ₃ ) and lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO ₃ ) to which additives such as Mg, Zn, Sc, In, and Fe are added using substrates of the same kind of material formation has been considered.

A ridge-type waveguide has a core formed on a base substrate according to a waveguide pattern, and has a step-type refractive index distribution. The core is in contact with air layers on three sides that are not in contact with the base substrate. A ridge waveguide can operate even if the top and sides of the core are air layers (with a refractive index of 1). However, as a practical problem, if the core layer is exposed, there is a concern that the characteristics may change with time due to adhesion of airborne dirt and dust. Moreover, in consideration of the mechanical resistance required for forming a film such as an AR coat on the end face of the optical waveguide, an over-cladding layer that also serves as a protective film may be provided.

On the other hand, the periodically poled structure is a structure for performing quasi-phase matching, and by inverting the crystal orientation for each coherence length of the fundamental wave and the wavelength-converted wave and reversing the sign of the nonlinear constant, phase inversion is achieved. This structure compensates for the amount of matching. It has high practical value in that it can perform wide wavelength conversion from the mid-infrared region to the visible region without using a special nonlinear optical crystal.

In general, the refractive index of a nonlinear optical material has temperature dependence, and in order to strictly satisfy the quasi-phase matching condition in a second-order nonlinear optical element, it is necessary to keep the temperature of the element constant. Normally, a thermometer such as a thermistor or a thermocouple is provided in or near the secondary nonlinear optical element to monitor its resistance value. A mechanism is provided to keep the element at a constant temperature using a temperature controller such as a heater or a Peltier element according to the monitor result.

However, there was a problem in precisely stabilizing the second-order nonlinear optical element with only the mechanism that controls the temperature controller so that the monitored value of the conventional thermometer is constant. This is because the temperature detector such as thermistor or thermocouple can monitor the average temperature of the entire secondary nonlinear optical element, and not the temperature of the waveguide portion itself that produces the nonlinear optical effect. . Therefore, it may not be possible to strictly operate at the optimum temperature simply by monitoring the temperature of the thermometer.

For example, assume that the temperature is controlled so that the secondary nonlinear optical element or a thermometer installed near it is constant. In the ridge waveguide, three side surfaces of the core, which is located on the surface of the device and which propagates light, are in contact with the air layer, but are not in contact with the base substrate. Therefore, when the environmental temperature of the device (outside air temperature) changes, the core of the ridge waveguide receives a slight change in the environmental temperature, resulting in a shift of the optimum operating point.

In addition, in order to obtain high conversion efficiency or high-gain optical parametric amplification, when strong pumping light is incident on the waveguide, heat is generated due to optical absorption of the pumping light that has entered the waveguide. This heat generation is local heat generation in the waveguide part, and it is difficult to correctly detect the shift of the optimum operating point due to the local heat generation only by monitoring the temperature detector installed in the element or its vicinity. Met.

Furthermore, since the amount of heat generated and the spatial distribution of heat generation differ depending on the method of use, it is necessary to search for the optimum operating point each time the method of use is changed. Specifically, when frequency down-conversion is used to generate harmonics from the fundamental wave, when frequency up-conversion is used to perform phase conjugate conversion for optical communication, and when frequency up-conversion is used to generate phase conjugate for optical communication, In the case of sensitive amplification, in the case of using frequency up-conversion to generate a two-photon entanglement state that is the basis of a messenger single-photon state, in the case of using frequency up-conversion to generate a vacuum squeezed state, each The levels of excitation light intensity and signal light intensity used in each case are different. As a result, the amount of heat generated and the spatial distribution of heat generation differ, making it difficult to achieve operation at the optimum operating point.

In order to solve this problem, a technique for adjusting the temperature using the fundamental wave light has also been abandoned (see, for example, Patent Document 1), but the situation where it can be applied is limited, and the fundamental wave light such as difference frequency generation is used. It cannot be applied in usages that do not use it.

As described above, the characteristics of devices that use second-order nonlinear optics are sensitive to the temperature of the element and easily change with the ambient temperature, making it difficult to stably operate at the optimum temperature using conventional methods. There was a problem. In addition, the charge amount of the element changes due to the change in the input signal intensity level, and the characteristic change due to the EO effect also poses a problem. Furthermore, depending on how the nonlinear optical device is used, for example, whether it is frequency up-conversion or frequency down-conversion, the difference in signal level in each wavelength band causes characteristic changes. Therefore, it is an issue to make wavelength conversion devices and optical parametric amplification devices using a second-order nonlinear optical element having a periodically poled structure exhibit stable and best characteristics regardless of the surrounding environment and the method of use.

JP 2015-225127 A

SUMMARY OF THE INVENTION An object of the present invention is to provide a wavelength conversion device that performs optical parametric amplification using a wavelength conversion element having high wavelength conversion efficiency. It is to express it by doing.

In order to achieve these objects, one embodiment of the present invention provides a nonlinear optical device comprising a second-order nonlinear optical element that generates converted light having different wavelengths from one or more fundamental wave lights, and the nonlinear optical device means for inputting monitor light having a wavelength different from the wavelength of the fundamental wave light into the nonlinear optical device; and output from the nonlinear optical device It is characterized by comprising a light detection means for separating monitor light and detecting light intensity, and a control means for controlling the temperature control device based on the light intensity detected by the light detection means.

FIG. 1 is a diagram showing the basic configuration of a wavelength conversion device according to a first embodiment of the present invention; FIG. 2 is a diagram showing an example of wavelength dependence of wavelength conversion efficiency in second harmonic generation of a nonlinear optical device; FIG. 3 is a diagram showing an example of the wavelength dependence of the wavelength conversion efficiency when the temperature of the element changes; FIG. 4 is a diagram for explaining error signal extraction in the wavelength conversion device of the first embodiment; FIG. 5 is a diagram for explaining selection of the wavelength of monitor light; FIG. 6 is a diagram showing the basic configuration of a wavelength conversion device according to a second embodiment of the present invention; FIG. 7 is a diagram showing the basic configuration of a wavelength conversion device according to a third embodiment of the present invention; FIG. 8 is a diagram for explaining error signal extraction in the wavelength conversion device of the third embodiment; FIG. 9 is a diagram showing the configuration of the wavelength conversion device according to the first embodiment; FIG. 10 is a diagram illustrating the configuration of a wavelength conversion device according to a second embodiment;

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, a wavelength conversion device that performs optical parametric amplification using a wavelength conversion element having high wavelength conversion efficiency will be described as an example.

[Basic concept]
FIG. 1 shows the basic configuration of the wavelength conversion device according to the first embodiment of the present invention. The wavelength converter 100 includes a nonlinear optical device 101 composed of an optical waveguide type second-order nonlinear optical element, and an optical multiplexer for inputting signal light, excitation light, and monitor light, which are fundamental wave lights, into the nonlinear optical device 101. 102 and an optical demultiplexer 103 for separating double waves of the signal light, excitation light and monitor light output from the nonlinear optical device 101 . A temperature control device that controls the temperature of the elements of the nonlinear optical device 101 is connected to a temperature control signal generation mechanism 104 and controlled by a feedback gain adjuster (PID controller) 105 . The feedback gain adjuster 105 detects the optical intensity of the double wave of the monitor light with the photodetector 106, and performs temperature control so that the conversion efficiency of the nonlinear optical device 101 is maximized.

FIG. 2 shows an example of the wavelength dependence of second harmonic generation in a nonlinear optical device. The phase matching wavelength at which the wavelength conversion efficiency of the nonlinear optical device 101 is maximized is 1544.70 nm when the element temperature is 45°C. The purpose is to adjust the temperature of the nonlinear optical device 101 so that wavelength conversion occurs at the phase-matched wavelength.

　Fig. 3 shows an example of the wavelength dependence of the wavelength conversion efficiency when the temperature of the element changes. It can be seen that the graph shifts left and right in response to changes in the temperature of the device without appreciably changing the function shape shown in FIG. It is an object of the present invention to suppress this shift amount by a control circuit.

The PDH method (Pound-Drever-Hall method) is generally used to control the system so that its response function always takes the maximum value. In the PDH method, a dither signal is generated in the system and a response signal is demodulated by the dither signal to obtain a differentiated function of the response function. By using this differential function as the error function, it is possible to lock at the point where the differential function becomes zero, that is, the point where the response function takes the maximum value. This utilizes the fact that the differential function becomes a monotonically decreasing or monotonically increasing function near zero.

In the wavelength conversion device of this embodiment, the response signal is the second harmonic generation efficiency for each wavelength. When using the PDH method, it is necessary to apply a dither signal to the wavelength conversion device, but in this embodiment the temperature control device is the only device to which a signal is applied to the system. The response speed of the temperature control device is slower than the signal handled by the wavelength conversion device, and the dither signal itself affects the operation of the system.

Therefore, in this embodiment, light having a wavelength slightly different from the phase-matched wavelength of the desired state is made incident on the nonlinear optical device, and an error signal is extracted from the response. Extraction of the error signal in the wavelength conversion device of this embodiment will be described with reference to FIG. As shown in FIG. 4A, as a target state, the wavelength conversion efficiency of second harmonic generation at a wavelength (monitor light wavelength) slightly different from the phase matching wavelength (signal light wavelength) is set to a target value (V _off ). and When the temperature of the element is low, as shown in FIG. _4B , an error signal (V _detect _−V _off )>0, the PID controller 105 controls the temperature control signal generation mechanism 104 to output a heating signal. When the temperature of the element is high, as shown in FIG. 4(c), the error signal (V _detect −V _off )<0, and the PID controller 105 outputs a cooling signal from the temperature control signal generation mechanism 104. to control.

Such a control method utilizes the fact that the conversion efficiency of monitor light is a monotonically decreasing or monotonically increasing function in the vicinity of the target state with respect to temperature changes. Selection of the wavelength of the monitor light will be described with reference to FIG. As shown in FIG. 5(a), when the target value is greater than the maximum value excluding the maximum value, temperature control is possible no matter how low the element temperature is, as long as the wavelength of the signal light is longer than that of the monitor light. is. Conversely, if the wavelength of the signal light is shorter than that of the monitor light, temperature control is possible no matter how high the element temperature is. On the other hand, as shown in FIG. 5(b), when the target value is smaller than the local maximum value excluding the maximum value, the temperature control range becomes narrow. Therefore, the wavelength of the monitor light should be closer to the phase-matched wavelength than the wavelength at which the minimum value is obtained, and should exhibit a wavelength conversion efficiency higher than all other maximum values in the function except for the maximum value. is desirable.

According to this embodiment, the monitor light is arranged near the phase matching wavelength. When the wavelength conversion device of this embodiment is applied to a wavelength division multiplexing (WDM) signal used in optical fiber communication, the phase-matched wavelength is within the guard band, so the operation of the WDM system is not affected. The advantage is that there is no

FIG. 6 shows the basic configuration of the wavelength conversion device according to the second embodiment of the present invention. In this configuration, the monitor light is input to the nonlinear optical device from the direction opposite to the direction of the signal light. The wavelength conversion apparatus 200 inputs a nonlinear optical device 201 comprising an optical waveguide type second-order nonlinear optical element, and signal light, excitation light, and monitor light, which are fundamental wave lights, into the nonlinear optical device 101 . and a circulator 203 for separating the signal light and excitation light output from the nonlinear optical device 201 and inputting the monitor light to the nonlinear optical device 201 . A temperature control device that controls the temperature of the elements of the nonlinear optical device 201 is connected to a temperature control signal generation mechanism 204 and controlled by a feedback gain adjuster (PID controller) 205 . A feedback gain adjuster 205 detects the light intensity of the converted light of the monitor light with a photodetector 206, and performs temperature control so that the conversion efficiency of the nonlinear optical device 201 is maximized.

FIG. 7 shows the basic configuration of the wavelength conversion device according to the third embodiment of the present invention. This is a configuration in which two lights generated on both sides of a phase-matched wavelength are used as monitor lights by utilizing left-right symmetry of the response function of second harmonic generation. Wavelength converter 300 includes a nonlinear optical device 301 composed of an optical waveguide type second-order nonlinear optical element, and signal light and excitation light, which are fundamental wave lights, and monitor light 1 and monitor light 2 are input to nonlinear optical device 301 . and an optical demultiplexer 303 for separating the signal light output from the nonlinear optical device 301, the excitation light, the second harmonic of the monitor light 1, and the second harmonic of the monitor light 2. . A temperature control device that controls the temperature of the elements of the nonlinear optical device 301 is connected to a temperature control signal generation mechanism 304 and controlled by a feedback gain adjuster (PID controller) 305 . The feedback gain adjuster 305 detects the light intensity of the converted light of the monitor light 1 and the monitor light 2 by the

photodetectors

306 and 307, and performs temperature control so that the conversion efficiency of the nonlinear optical device 301 is maximized.

Extraction of the error signal in the wavelength converter of the third embodiment will be described with reference to FIG. As shown in FIG. 8A, since the response function of the wavelength conversion efficiency of second harmonic generation is symmetrical, two wavelengths (monitor light wavelength 1, monitor light wavelength 2 ). The two wavelengths are wavelengths slightly different from the phase matching wavelength (signal light wavelength) and have the same value of wavelength conversion efficiency, and these values are set as target values (V _detect1 , V _detect2 ). When the temperature of the element is low, as shown in FIG. 8B, the error signal (V _detect1 −V _detect2 ), which is the difference between the two, becomes >0, and the PID controller 305 causes the temperature control signal generation mechanism 304 to heat. Control so that the signal is output. When the temperature of the element is high, as shown in FIG. 8(c), the error signal (V _detect1 −V _detect2 )<0, and the PID controller 305 outputs a cooling signal from the temperature control signal generation mechanism 304. to control.

With such a control method, it is not necessary to know the target value as long as it is known that the response function of the wavelength conversion efficiency of second harmonic generation is bilaterally symmetrical about the phase matching wavelength.

The monitor light may be generated by light from a light source that is completely different from the signal light, or by splitting a part of the signal light or pump light and subjecting it to intensity modulation, as will be described later. You may

FIG. 9 shows the configuration of the wavelength conversion device according to the first embodiment. The wavelength conversion device 400 includes a nonlinear optical device 401 having a periodically poled lithium niobate (PPLN) waveguide, a signal light input to the nonlinear optical device 401, and a monitor light from the nonlinear optical device 401. and a circulator 403 for separating the signal light output from the nonlinear optical device 401 and inputting the monitor light from the monitor light source 409 to the nonlinear optical device 401 . The excitation light enters nonlinear optical device 401 along with signal light via dichroic mirror 407 and is separated from the output of nonlinear optical device 401 via dichroic mirror 408 . A Peltier element 411 that is a temperature control device of the nonlinear optical device 401 is connected to a temperature control signal generation mechanism 404 and controlled by a feedback gain adjuster (PID controller) 405 . A feedback gain adjuster 405 detects the optical intensity of the converted light of the monitor light with a photodetector 406, and performs temperature control so that the conversion efficiency of the nonlinear optical device 401 is maximized.

The nonlinear optical device 401 has a periodically poled structure that satisfies quasi-phase matching between an input optical signal and pumping light, and output converted light, and is a wavelength converter and optical parametric amplifier with high wavelength conversion efficiency. works as In the PPLN waveguide, the quasi-phase matching condition is satisfied among the three waves of pumping light, signal light and conversion light. In other words, if the effective refractive indices in the waveguides of the excitation light, signal light, and converted light are respectively np, ns, and nc, then
np/λp−ns/λs−nc/λc=1/Λ (Formula 4)
It has a polarization-inverted structure with an inversion period Λ that satisfies

A wavelength division multiplexing (WDM) signal composed of optical signals of a plurality of wavelengths is input to the wavelength conversion device 400 as signal light. In the nonlinear optical device 401, the WDM signal combined with the excitation light is incident on the PPLN waveguide, and the WDM signal is converted by difference frequency generation.

Here, the fundamental wave light wavelength λ0 (frequency: ω0) was set to 1545.00 nm, and the excitation light wavelength λp (frequency: 2ω0) was set to 772.5 nm. The monitor light was input to the nonlinear optical device 401 in the opposite direction to the signal light, and had a wavelength of 1545.10 nm. The monitor light is converted into light with a wavelength of 772.55 nm by second harmonic generation in the PPLN waveguide of the nonlinear optical device 401 and is input to the photodetector 406 via the dichroic mirror 407 and circulator 402 .

By inputting excitation light and signal light into the nonlinear optical device 401, conversion light is generated by difference frequency generation in the PPLN waveguide. For example, if the signal light wavelength λs (frequency: ωs) is 1540 nm, 2ω0−ωs generates converted light with a wavelength of 1550 nm. Converted light is generated in a folded form on the wavelength axis with the fundamental light wavelength λ0 as the center.

In the first embodiment, light whose wavelength is slightly shifted from the signal light is used as monitor light. In general, the efficiency of second-order harmonic generation of a second-order nonlinear optical medium having a uniform periodically poled structure takes the form of the square of the sinc function with respect to incident light in the fundamental waveband. This function has a shape as shown in FIG. 2, and the maxima appearing on the sides are relatively suppressed compared to the maxima at the center where the efficiency is the highest. As a criterion for determining the wavelength of the monitor light, a wavelength that exists within the peak including the central maximum value in this function and has a higher conversion efficiency than all other maximum values excluding the maximum value should be selected. .

In the vicinity of the target value, the light intensity detected by the photodetector 406 decreases as the temperature increases and increases as the temperature decreases. This change is input to the feedback gain adjuster 405 as an error signal and fed back to the control current of the temperature control signal generating mechanism 404 . As a result, the intensity of wavelength-converted light could be stabilized within 0.2 dB over the entire band.

Note that the monitor light may be separated from the signal light by using an optical wavelength multiplexer/demultiplexer without using the optical circulator, and the same effect was obtained with this configuration. Furthermore, when an optical wavelength multiplexer/demultiplexer is used, the monitor light may be incident from the same direction as the signal light, and similar effects can be obtained with this configuration.

FIG. 10 shows the configuration of the wavelength conversion device according to the second embodiment. The wavelength converter 500 includes a nonlinear optical device 501 having a periodically poled lithium niobate (PPLN) waveguide, and a signal light, monitor light 1 and monitor light 2 input to the nonlinear optical device 501 . An optical multiplexer 502, a dichroic mirror 507 that multiplexes the excitation light and inputs it to the nonlinear optical device 501, a dichroic mirror 508 that separates the signal light output from the nonlinear optical device 301, and further excitation light and monitor light. and an optical demultiplexer 503 for separating the double wave of 1 and the double wave of the monitor light 2 . A Peltier element 511 , which is a temperature control device for the nonlinear optical device 501 , is connected to a temperature control signal generation mechanism 504 and controlled by a feedback gain adjuster (PID controller) 505 . The feedback gain adjuster 505 detects the optical intensity of the double wave of the monitor light with the

photodetectors

561 and 562, obtains the difference from the differentiator 563, and maximizes the conversion efficiency of the nonlinear optical device 501. , temperature control.

A part of the signal light is split by the optical splitter 521, and the spectrum of only the fundamental wave is extracted by the wavelength filter 522. A modulator 523 applies intensity modulation of 10 GHz to the extracted fundamental wave spectrum light to generate two monitor lights with wavelengths separated by about 0.1 nm on both sides of the fundamental wave light wavelength. Here, the fundamental wave light wavelength λ0 (frequency: ω0) is 1545.00 nm, the excitation light wavelength λp (frequency: 2ω0) is 772.50 nm, monitor light 1 is the monitor light with a shorter wavelength than the fundamental wave, and monitor light 1 is the monitor light with a shorter wavelength than the fundamental wave. The long monitor light is referred to as monitor light 2 . Monitor light 1 and monitor light 2 are input to nonlinear optical device 501 via optical multiplexer 502 .

The nonlinear optical device 501 has a periodically poled structure that satisfies quasi-phase matching between an input optical signal and pump light, and output converted light, and is a wavelength converter and optical parametric amplifier with high wavelength conversion efficiency. works as

Monitor light 1 and monitor light 2 whose frequency has been doubled are output to the output side of the nonlinear optical device 501, and are received by

separate photodetectors

561 and 562, respectively, and a difference signal from a differentiator 563 is fed back. Input to gain adjuster 505 . In the vicinity of the target value, the light intensity detected by the photodetector 561 decreases as the temperature increases and increases as the temperature decreases. In the vicinity of the target value, the light intensity detected by the photodetector 562 increases as the temperature increases and decreases as the temperature decreases. Since this difference becomes a monotonic function near the target value, it can be used as an error signal. This error signal is input to the feedback gain adjuster 505 and fed back to the control current of the temperature control signal generating mechanism 504 . As a result, the intensity of wavelength-converted light could be stabilized within 0.2 dB over the entire band.

In general, the efficiency of second-order harmonic generation of a second-order nonlinear optical medium having a uniform periodically poled structure is in the form of the square of the sinc function for incident light in the fundamental wave band, and the wavelength of the fundamental wave is The shape is symmetrical with respect to the center. By generating a control signal such that the output of the photodetector 561 and the output of the photodetector 562 are equal, that is, the difference is zero, the target state can be stably achieved. If the shape of the function becomes asymmetrical due to manufacturing errors, it is possible to adjust to the optimum point by adding an offset to the error signal.

Although a PPLN waveguide is used in this embodiment, a waveguide that does not have a periodically poled structure may also be used, and if the material has a second-order nonlinear optical coefficient, it does not need to be lithium niobate. good. Also, even when two monitor lights are used, the monitor light may be input to the nonlinear optical device in the opposite direction to the signal light by combining a circulator and a wavelength multiplexer/demultiplexer.

Claims

A wavelength conversion apparatus comprising a nonlinear optical device comprising a second-order nonlinear optical element that generates converted light having different wavelengths from one or more fundamental light beams, and a temperature control device that controls the temperature of the elements of the nonlinear optical device,
means for inputting monitor light having a wavelength different from the wavelength of the fundamental wave light into the nonlinear optical device;
a photodetector for separating the monitor light output from the nonlinear optical device and detecting the light intensity;
and control means for controlling the temperature control device based on the light intensity detected by the light detection means.
The wavelength of the monitor light is set so that the second-order nonlinear optical element generates the second-order harmonic most efficiently with respect to the fundamental-wave light, and the conversion efficiency of second-order harmonic generation is the wavelength of the fundamental-wave light. 2. The wavelength conversion device according to claim 1, wherein the function is set to be a monotonic function between the wavelength of the monitor light and the wavelength of the fundamental wave light.
3. The wavelength converter according to claim 2, wherein the wavelength of said monitor light is set to be a wavelength with a higher conversion efficiency than all other maximum values within said function except for the maximum value. .
the monitor light is two monitor lights of a first monitor light wavelength and a second monitor light wavelength;
the first monitor light wavelength and the second monitor light wavelength are set so that the conversion efficiencies are the same across the maximum value in the function;
3. The wavelength conversion apparatus according to claim 2, wherein said control means controls said temperature control device based on a difference in light intensity of said two monitor lights.
The wavelength conversion device according to any one of claims 1 to 4, wherein the monitor light is generated by branching a part of the fundamental wave light and performing intensity modulation.
6. The wavelength conversion apparatus according to claim 1, wherein the monitor light is input to the nonlinear optical device from a direction opposite to that of the fundamental wave light input to the nonlinear optical device. .
The wavelength converter according to any one of claims 1 to 6, wherein the nonlinear optical device has a periodically poled lithium niobate (PPLN) waveguide.